While seemingly simple in theory, this has me stumped! I'm looking to find the average days between a customer's transactions recorded on a transactional 'ChangeDate'. I'm sure it has something to do with Lookup, but was unsure how to implement it in this scenario.

Datediff('day',Lookup([ChangeDate], 1), [ChangeDate]) - gives the aggregation error and I'm not sure if the 1 offset will even work when the entries are so spread out.